Temperature sensors have multiple applications in a system-on-a-chip (SoC) integrated circuit. For example, a processor temperature sensor monitors the central processing unit (CPU) temperature so that the CPU operation may be throttled to avoid exceeding the thermal threshold for the CPU. Similarly, dynamic voltage scaling for the CPU power supply voltage and clocking frequency may be controlled responsive to a processor temperature sensor. In addition, a temperature sensor may be integrated with precision analog circuitry within an integrated circuit so that temperature effects may be compensated.
However, prior art temperature sensors have a number of issues with regard to their implementation. For example, bipolar junction transistors have commonly been used to form a temperature sensor. But bipolar transistors in modern complementary metal-oxide semiconductor (CMOS) processing nodes are parasitic devices that are relatively large and have increased variability as compared to metal-oxide semiconductor field-effect transistor (MOSFET) devices. Sub-threshold MOSFET temperature sensors have thus been developed to avoid the use of bipolar junction transistors. A conventional sub-threshold MOSFET temperature sensor 100 is shown in FIG. 1. An n-type metal-oxide semiconductor (NMOS) sub-threshold transistor M2 has its source connected to an output node for the output voltage Vout. The gate of sub-threshold transistor M2 is also connected to the output node so that sub-threshold transistor M2 has a gate-to-source voltage of zero volts. Sub-threshold transistor M2 will thus conduct only a sub-threshold leakage current. The drain of sub-threshold transistor M2 connects to a power supply voltage node. An NMOS diode-connected transistor M1 has its gate and drain connected to the output node and its source connected to ground. Diode-connected transistor M1 will thus conduct the sub-threshold leakage current conducted by sub-threshold transistor M2. The diode connection for diode-connected transistor M1 develops the output voltage on the output node in response to the sub-threshold leakage current from sub-threshold transistor M2. Sub-threshold transistor M2 would typically be many times larger than diode-connected transistor M1 so that a sufficient amount of sub-threshold leakage current is passed to produce the output voltage at the drain of diode-connected transistor M1.
Although sub-threshold MOSFET temperature sensor 100 avoids the problems resulting from bipolar junction transistor variability and die area demands, its operation still suffers from a number of problems. For example, the bulk-to-source voltages for diode-connected transistor M1 and sub-threshold transistor M2 are not matched, which leads to variability for the resulting temperature sensor performance. In addition, the drain-to-source voltage (Vds) for sub-threshold transistor M2 is relatively large. Such a large Vds provokes additional leakage mechanisms such as gate-induced drain leakage (GIDL) effects that spoil the desired proportional-to-absolute temperature (PTAT) behavior of the sub-threshold MOS temperature sensor 100. Accordingly, there is a need in the art for improved sub-threshold MOSFET temperature sensors.